The age dependence of elicitation of adjuvant arthritis in three different strains of rats is recorded. In contrast to the existing literature an adjuvant disease can be elicited already in animals of 13 to 15 days of age, where significant, obvious genetical differences exist between the used strains with regard to age dependence of the occurence of arthritis. From the results is to conclude, that in all investigations on adjuvant arthritis merely genetically identic animals are to be used, and the possibility of strain dependent differences in the assessment of the results are to be considered.